In recent years, a porous membrane is utilized in a variety of areas, for example, a water treatment field such as water purification treatment and wastewater treatment, a pharmaceutical and medical field such as drug manufacture and blood purification, a fermentation industry field, a food industry field, a battery separator, a charged membrane, and an electrolyte membrane for fuel cells.
Among others, in the drinking water production field and industrial water production field, i.e., in the water treatment field such as usages for water purification treatment, wastewater treatment and seawater desalination, a porous membrane is used as an alternative to conventional sand filtration, coagulating sedimentation and evaporation methods or for enhancing the quality of treated water. In these fields, since the amount of water to be treated is large, a porous membrane with excellent water permeation performance makes it possible to reduce the membrane area, save the equipment cost due to a compact apparatus, and is advantageous in view of membrane exchange cost or installation area.
As the porous membrane for water treatment, a membrane appropriate to the size of a separation target substance contained in the water to be treated is used. Usually, natural water contains many suspended components, and a separation membrane such as microfiltration membrane or ultrafiltration membrane for the removal of suspended components in water is therefore used in general.
In the water treatment, for the purpose of sterilizing permeate or preventing biofouling of separation membrane, a sterilizer such as sodium hypochlorite may be added to the portion of separation membrane module, or as the chemical cleaning of separation membrane, the separation membrane may be washed with an acid such as hydrochloric acid, citric acid and oxalic acid, an alkali such as aqueous sodium hydroxide solution, chlorine, a surfactant, etc. Accordingly, a separation membrane using, as a material having high chemical resistance, a fluororesin-based polymer typified by polyvinylidene fluoride has been recently developed and utilized.
In the water purification treatment field, a problem of a chlorine-resistant pathogenic microorganism such as cryptosporidium getting mixed in with drinking water has been manifested since late 20th century, and it is required for the porous hollow-fiber membrane to have high strength so that no mixing of raw water is caused by membrane breakage.
In order to obtain a porous hollow-fiber membrane having high water permeation performance, high strength/elasticity, and high chemical resistance, various methods have been heretofore proposed.
For example, Patent Document 1 discloses a melt-extraction method. In Patent Document 1, two kinds of fluororesin-based polymers differing in the weight average molecular weight are used and after adding a plasticizer and a good solvent thereto, the resulting mixture is melt-extruded into a hollow-fiber membrane shape, cooled/solidified, subjected to extraction of the plasticizer, and stretched to obtain a porous hollow-fiber membrane in which a mixture of a crystal oriented portion and a crystal unoriented portion is observed.
Patent Document 2 discloses a method in which a fluororesin-based polymer solution containing a fluororesin-based polymer and a poor solvent therefor and being at a temperature not less than the phase separation temperature is discharged into a cooling bath at a temperature not more than the phase separation temperature and solidified to obtain a hollow-fiber membrane.
Furthermore, in Patent Document 3, a fibrous texture having a diameter of 0.9 μm to 3 μm and being oriented in the length direction of a porous hollow-fiber membrane including a fluororesin-based polymer accounts for 30% or more of the entire porous hollow-fiber membrane, whereby a porous hollow-fiber membrane excellent in strength and pure-water permeation performance is obtained.